Apparatus for the continuous dehydration of aluminum fluoride hydrates

ABSTRACT

Apparatus for the continuous dehydration of aluminum fluoride hydrates consisting of a fluidized-bed furnace with a cylindrical furnace jacket and a perforated plate just above the base, a lid to which gas-heated jacket radiant tubes are fixed reaching vertically almost to the said plate each of the tubes being closed at one end and having a concentric inner tube with a burner in the interior and being of almost the same length as the jacket radiant tube as well as open at the bottom so that combustion gas issuing from said burner in the interior of said inner tube flows upward in the tip of the outer tube after a change of direction of 180*. Thus both the fluidized bed outside the jacket radiant tube and, via a recuperator, the air passed to the burner are warmed.

United States Patent Inventors Alfred Schmidt;

Ferdinand Weinrotter; Roland Glotzl;

Rudolf Staudigl, all of Lint/Danube,

APPARATUS FOR THE CONTINUOUS DEHYDRATION OF ALUMINUM FLUORIDE HYDRATES 6 Claims, 2 Drawing Figs.

US. Cl 263/21 A, 23/284, 34/57 A, 126/91 A Int. Cl F27b 15/00 Field of Search 263/21, 21

A; 34/57 A; 126/91 A; 23/284 Primary Examiner-Charles J. Myhre Armrney-Wenderoth. Lind 8: Ponack ABSTRACT: Apparatus for the continuous dehydration of aluminum fluoride hydrates consisting of a fluidized-bed furnace with a cylindrical furnace jacket and a perforated plate just above the base. a lid to which gas-heated jacket radiant tubes are fixed reaching vertically almost to the said plate each of the tubes being closed at one end and having a concentric inner tube with a burner in the interior and being of almost the same length as the jacket radiant tube as well as open at the bottom so that combustion gas issuing from said burner in the interior of said inner tube flows upward in the tip of the outer tube after a change of direction of 180. Thus both the fluidized bed outside the jacket radiant tube and. via a recuperator. the air passed to the burner are warmed.

PATENTEDunv 2 Ian 3,617; 038

I'NVENTORS ALFRED SCHMIDT FERDINAND WEINROTTER ROLAND GLC'DTZL RUDOLF STAUDIGL BY Wm MWZW ATTORNEYS APPARATUS FOR THE CONTINUOUS DEIIYDRATION F ALUMINUM FLUORIDE IIYDRATES This invention relates to an apparatus for the continuous dehydration of aluminum fluoride hydrates.

It is known from British Pat. specification No. 91 1,857 that a fluidized bed device with external heating can be employed for the dehydration of aluminum fluoride hydrates. In the device disclosed in this prior specification it is important that the hydrate to be dehydrated should be brought to the desired temperature within a few seconds. This is because only then can a splitting off of hydrogen fluoride by hydrolysis during the heating process be avoided and hence an aluminum fluoride of high purity be obtained.

The calcination of moist aluminum fluoride hydrate requires an amount of heat of about 1,150 Kcal. per kilogram of final product. Since the requisite final temperature during the calcination is about 600 C. it is necessary for the walls, which after all are intended to transfer the heat to the fluidized bed particles, to be at a temperature of 650 to 800 C.

However, only few technically usable materials still have the requisite chemical and mechanical resistance at these temperatures. Thus it is only high-alloy chromium-nickel steels which can still be considered as usable materials. Thesehowever suffer from the great disadvantage of increased brittleness, especially at curved and welded positions, which particularly exerts an effect at the weld positions on the heated wall, where thermal expansions, as a result of the diameter of the apparatus, play a significant role.

These disadvantages become significant if the plant is in creased in size, coupled with an increase in size of the diameter of the fluidized bed furnace. Furthermore, given a greater diameter, it is no longer assured that the entire product is brought to a temperature of 600 C. within a few seconds.

A device for the calcination of aluminum fluoride has to meet the requirement of accommodating the requisite heating surface while the volume is as small as possible and hence the residence time of the product is short, and has to take into account the properties of the available materials.

Direct heating of the fluidized bed is excluded for chemical reasons and the amount of heat which can be supplied by superheating the fluidization gas is inadequate for the present case, so that the principle of heating by means of heated surfaces cannot be abandoned.

It is also already known to heat fluidized beds by means of heaters, so-called radiant tubes, with a gas or oil burner at one end, which project into the fluidized bed. Thus for example a device has been proposed in which a radiant tube heated by gas or oil outside the fluidized bed passes through the fluidized bed in two loops, with the bends being in each case outside the fluidized bed. This requires that the radiant tube has for example to be carried in the furnace jacket at six points and as a result of the tube being more strongly heated than the furnace jacket itself, severe stresses occur at these passage points. Furthermore, fluidized beds have also been heated by combustion chambers located directly in the fluidized bed, leading to severe local overheating in the combustion chamber relative to the gas inlets and outlets.

Jacket radiant tubes have become known in the metal industry for heat treatment furnaces, for example, flow-through furnaces, that is to say for heating the atmosphere of the furnace. Such a jacket radiant tube consists of a tube closed at one end, inside which a second tube is arranged concentrically in such a way that an annular space is formed between the two tubes. The burner provided in the interior of the inner tube is so constructed that it simultaneously serves as a recuperator, and the combustion gases are led in such a way that their direction is changed through 180 at the end of the inner tube and that they flow back in the annular space, flush round the recuperator burner and hence prewarm the air used for combustion. As a result the temperature remains practically constant over the length of the tube because nowhere does a directly heated surface serve as an outer wall; rather, the outer wall is internally flushed by the combustion gases.

It has now surprisingly been found that such jacket radiant tubes are particularly suitable for the rapid and uniform heating of fluidized beds to a very high temperature, as is required for the successful calcination of aluminum fluoride hydrates.

Thus, according to the present invention there is provided an apparatus for the continuous dehydration of aluminum fluoride hydrates, which comprises one or more fluidized bed furnaces, the furnaces being arranged in series when there are more than one, each individual furnace consisting of a cylindrical furnace jacket which is provided, just above the base, with a porous perforated plate or sintered plate, and has a furnace lid to which gas-heated jacket radiant tubes are fixed which project vertically into the interior of the furnace and reach almost to the perforated or porous sintered plate, each of the radiant tubes being closed at one end and having arranged in the interior a concentric inner tube which is open at the bottom and is of almost the same length as the jacket radiant tube, so that an annular space is formed between the two tubes through which combustion gas issuing from a burner arranged in the interior of the concentric inner tube flows upwards after a change of direction of in the tip of the outer tube, so that both the fluidized bed located outside the jacket radiant tube and also, via a recuperator, the air passed to the burner are warmed.

A particularly preferred apparatus according to the invention is one in which there are several jacket radiant tubes which are of the same diameter and are spaced equally from one another and are arranged in the furnace in one or more circles concentric with the furnace jacket. A uniform distribution of heat in the fluidized bed is thereby achieved.

The invention will be more particularly described with reference to the accompanying drawings in which:

FIG. 1 and FIG. 2, schematically illustrate a particularly preferred embodiment of the apparatus according to the invention. FIG. 1 represents the elevation and FIG. 2 the corresponding plan view.

In the accompanying drawings, each individual fluidized bed furnace essentially consists of a cylindrical furnace jacket 1. The fluidized bed furnace is closed at the top by a flat lid 2 with apertures for the product inlet tube 6, for one or more jacket radiant tubes 5 and for a gas outlet tube 7. The apparatus includes an inlet tube 13 for the heating gas, an inlet tube 12 for the combustion air, and a gas discharge pipeline 14 for the combustion gas. The jacket radiant tube 5, has a burner 11 which is surrounded by a recuperator 15. The direction of the combustion gases is changed through 180 at the end of the inner tube 16, and the combustion gases flow back in the direction of the arrow and flush round the recuperator.

In a particularly preferred embodiment three or four jacket radiant tubes 5 are arranged in a circle which is concentric with the furnace jacket. The furnace is closed at the bottom by a base 3 in which is set a perforated or porous sintered plate 4. The blowing gas enters at 9. At the side of the product outlet tube 8 there is a curved screening sheet 10 which starts just below the gas outlet tube 7, reaches almost to the sintered plate 4, and is firmly connected to the furnace jacket 1. Screening sheet 10 is a gastight sheet, preferably of metal, positioned to channel only that product which has reached the bottom of the furnace. Therefore, as will be apparent from the drawings, only that product which has undergone the entire process may be discharged.

In order to carry out the dehydration of aluminum fluoride hydrates, the blowing gas, which flows in through the porous sintered plate 4, is used to create a fluidized bed of anhydrous aluminum fluoride in the apparatus according to the invention, this bed being kept at a temperature of 580 to 600 C. with the aid of the jacket radiant tubes 5 if the calcination is being carried out in a single fluidized bed furnace. The moist aluminum fluoride hydrate is introduced into the furnace from above through the product inlet tube 6 built into the lid 2 and reaches the desired final temperature extremely rapidly because, as a result of being introduced from above, it immediately comes into contact with the hot fluidized bed and with the jacket radiant tubes. In order to avoid incompletely dehydrated aluminum fluoride leaving the furnace at 8, the product outlet tube 8 is so screened by the screening sheet 10 that only products which were already in the lowest part of the fluidized bed can be expelled. If a battery of, for example, two fluidized bed furnaces are employed for the dehydration, it is advisable to carry out the dehydration in two stages, with the first fluidized bed furnace for example being kept at about 220 C., corresponding to a partial dehydration of the trihydrate with the release of 2.5 moles of water, and only the second furnace being kept at 580 to 600 C., the temperature for complete dehydration. in both furnaces the product is brought to the desired temperature within a few seconds. The product outlet tube 8 of the first furnace is then connected to the product inlet tube 6 of the second furnace.

We claim:

1. An apparatus for the continuous dehydration of aluminum fluoride hydrates and comprising a fluidized bed furnace consisting of a cylindrical furnace jacket having a base; a gas-permeable plate positioned within said jacket immediately above said base, a fluidized bed adapted to be positioned within said jacket above said base; a furnace lid over the top of said jacket; at least one gas-heated jacket radiant tube extending through said lid and projecting vertically downwardly into the interior of said furnace and reaching almost to said gaspermeable plate; each of said gas-heated jacket radiant tubes consisting of an outer tube closed at the lower end near said gas-permeable plate, an inner tube concentrically arranged within said outer tube, the upper part of said inner tube being constructed to serve as a recuperator, said inner tube being open at the lower end thereof and reaching almost the closed end of said outer tube, and a burner positioned within said inner tube, whereby an annular space is formed between said outer tube and said inner tube; a source of heating gas connected to said burner; a source of combustion air connected to the interior of said inner tube opposite said open end; and combustion gas discharge means connected to said annular space opposite said lower end of said outer tube; the arrangement being such that said heating gas and combustion air are ignited by said burner, and said resultant combustion gas flows downwardly through said inner tube, and then upwardly through said annular space to said discharge means; whereby said fluidized bed without said outer tube is heated and said combustion air within said inner tube is heated via said recuperator.

2. An apparatus as claimed in claim 1, wherein said gaspermeable plate is a perforated plate.

3. An apparatus as claimed in claim 1, wherein said gaspermeable plate is a porous sintered plate.

4. An apparatus as claimed in claim 1, wherein said apparatus comprises a plurality of said fluidized-bed furnaces, each of said furnaces having a product inlet and a product outlet, said plurality of furnaces being serially connected such that the product outlet of a first of said furnaces is connected to the product inlet of a second of said furnaces.

5. An apparatus as claimed in claim 1, in which several jacket radiant tubes which have the same diameter and are equally spaced from one another are arranged in said furnace in one or more circles concentric to said furnace jacket.

6. An apparatus as claimed in claim 1, further comprising a product inlet and a gas outlet in said lid, a product outlet in said jacket, a blowing gas supply aperture in said base, and a curved screening sheet positioned to extend from just below said gas outlet to just above said gas-permeable plate and connected in a gastight manner to said jacket around said product outlet, whereby only product from the bottom of said furnace is channeled to said product outlet.

t i i 

2. An apparatus as claimed in claim 1, wherein said gas-permeable plate is a perforated plate.
 3. An apparatus as claimed in claim 1, wherein said gas-permeable plate is a porous sintered plate.
 4. An apparatus as claimed in claim 1, wherein said apparatus comprises a plurality of said fluidized-bed furnaces, each of said furnaces having a product inlet and a product outlet, said plurality of furnaces being serially connected such that the product outlet of a first of said furnaces is connected to the product inlet of a second of said furnaces.
 5. An apparatus as claimed in claim 1, in which several jacket radiant tubes which have the same diameter and are equally spaced from one another are arranged in said furnace in one or more circles concentric to said furnace jacket.
 6. An apparatus as claimed in claim 1, further comprising a product inlet and a gas outlet in said lid, a product outlet in said jacket, a blowing gas supply aperture in said base, and a curved screening sheet positioned to extend from just below said gas outlet to just above said gas-permeable plate and connected in a gastight manner to said jacket around said product outlet, whereby only product from the bottom of said furnace is channeled to said product outlet. 